Lying is intrinsically evil. Every intrinsically evil act is always immoral because the act is, by its very nature, contrary to the moral law. But not every intrinsically evil act is a mortal sin. Some intrinsically evil acts are venial sins; other intrinsically evil acts are mortal sins.

Lying is sometimes a venial sin and other times a mortal sin. The three fonts of morality determine if a sin is venial or mortal. If any one or more fonts is gravely disordered, meaning gravely contrary to the love of God or the love of neighbor as self, then the act is objectively a mortal sin. If the act is a sin, but none of the fonts of morality are gravely disordered, then the sin is only venial.

The three fonts [or sources] of morality are: (1) intention, (2) moral object, (3) circumstances. If any one font is bad, the act is a sin. If all three fonts are good, the act is moral.

Lying is intrinsically evil, and therefore it is immoral due to its moral object, in the second font. Lying is a sin regardless of intention or circumstances because it has an evil moral object. So even if the intention is good, or the consequences (in the circumstances) are good, the act is still a sin.

(1) If a lie includes a gravely disordered intended end, such as sending an innocent person to prison, doing serious harm to the reputation of another person, or any other gravely immoral intention, then the lie becomes a mortal sin, due to the intention.

(2) If the lie has a gravely immoral object, then the lie would be a mortal sin. The gravity of the disorder in the object depends on the moral weight of the truth being denied, or distorted (including the extent of the distortion). When the denial of a particular truth, or the distortion of a particular truth, is entirely incompatible with the love of God and neighbor, then it is a mortal sin due to a gravely immoral object. For example, to lie about a grave matter of faith or morals would have a gravely disordered object. Whereas to lie to someone about a small matter would have a disordered object, but would not be gravely disordered. So-called ‘white lies’ are generally venial sins (unless the term is being misapplied).

If the act of lying is a venial sin, having nothing gravely disordered in any font, the act becomes a mortal sin if there is a second moral object, one that is gravely immoral. For example, to lie about a small matter is usually venial sin. But if one were to lie about that same small matter while under oath (in any relevant matter), the lie becomes a mortal sin due to the second moral object, the breaking of an oath. In other words, a venial lie becomes a mortal lie because it is also perjury.

(3) If the bad consequences of the lie will gravely outweigh, in terms of morality, the good consequences, as these consequences can be reasonably anticipated at the time that the lie is told, then the lie would be a mortal sin due to the circumstances.

“The gravity of a lie is measured against the nature of the truth it deforms, the circumstances, the intentions of the one who lies, and the harm suffered by its victims. If a lie in itself only constitutes a venial sin, it becomes mortal when it does grave injury to the virtues of justice and charity.” (CCC, n. 2484).

The nature of the truth that a lie deforms pertains to the moral object in the second font. The harm suffered by anyone due to the lie, in so far as this harm can be reasonably anticipated, is in the circumstances, the third font. The intentions of the person who lies is in the first font. So the gravity of a lie depends on the three fonts of morality.

Most of the lies that people often tell in their lives, we may charitably assume, are venial lies. No number of venial sins can add up to a mortal sin. But the more that one commits venial sin without repentance, the closer one moves to eventually committing a mortal sin. Also, if a person continually tells venial lies, with no regard for truth whatsoever, he may well have committed a mortal sin, interiorly, by completely abandoning truth — even if each and all of his particular lies are venial.

[Ephesians]
{4:25} Because of this, setting aside lying, speak the truth, each one with his neighbor. For we are all part of one another.